Exhaust gas analyzer for internal combustion engines

ABSTRACT

AN EXHAUST GAS ANALYZER WHICH COMBINES THE KNOWN MEASURE OF EXHAUST GASES BASED UPON THE THERMAL CONDUCTIVITY OF THE EXHAUST GASES AND THE KNOWN MEASURE BASED UPON THE CATALYTICAL BURNING OF THE EXHAUST GASES INTO A NOVEL SYSTEM, ACCORDING TO WHICH THE OUTPUT SIGNALS OBTAINED BY THESE TWO MEASUREMENTS, MADE SIMULTANEOUSLY, ARE SUBTRACTED FROM EACH OTHER. THIS RESULTS IN AN ANALYZER WHICH HAS AN UNEQUIVOCAL INDICATION IN THE REGION BELOW AND ABOVE THE STOICHIOMETRICAL POINT.

July 4, 1972 5 Sheets-Sheet} Filed Aug. 22, 1969 FIG. 1A

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BY WI/IHMIIW July 4, 1972 H. R. GEUL 3,674,436

EXHAUST GAS ANALYZER FOR INTERNAL COMBUSTION ENGINES Filed Aug. 22, 1969 3 Sheets-Sheet 2 Fl G 3A INVENTOR. A-zmw P. 6201.

July 4, 1972 H. R. GEUL 3,674,436

EXHAUST GAS ANALYZER FOR INTERNAL COMBUSTION ENGINES Filed Aug. 22, 1969 3 Sheets-Sheet 5 F l G 4 INVENTOR. f 2 6&0!

United States Patent 3,674,436 EXHAUST GAS ANALYZER FOR INTERNAL COMBUSTION ENGINES Herman R. Geul, Rossinistraat 12, Leiden, Netherlands Filed Aug. 22, 1969, Ser. No. 852,300 Int. Cl. G01n 25/20, 25/36, 33/22 US. Cl. 23-232 E 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION For the purpose of analysis of the exhaust gases of internal combustion engines two different systems exist, based upon two different principles. According to the first system, an element heated by a certain energy supply and washed by the gas to be analyzed is used, whereby, as a result of the fact that the thermal conductivity of the gas mixture to be analyzed varies as a function of its compositon, the resulting final temperature of the element depends on the composition of this gas mixture. The final temperature can be detected, for instance, by means of a suitable sensor, which provides an output signal representing the composition of the gas mixture.

The thermal conductivity of the mixture is minimal when the combustion takes place stoichiometrically in the internal combustion engine, i.e. when the air to fuel ratio of the mixture, fed to the engine, is such that a complete combustion takes place. In that case, the exhaust gases contain hardly any CO and H but mainly CO nitrogen and water vapour, the latter being removed by condensation.

Above and below this point the thermal conductivity increases, the element is cooled better and, as a result, its temperature falls. It is thus clear that two particular values of the air to fuel ratio, respectively, lying above and below the stoichiometrical point, give the same indications on the measuring instrument. In other words, around the stoichiometrical point the indication of the measuring instrument is not unequivocal. It then is impossible to ascertain whether the value of the air to fuel ratio is too large or too small, i.e., whether the mixture, fed to the cylinders, is lean or rich. In modern engines in which an exact adjustment of the composition of the fuel mixture at no load or part-load is essential, not only to avoid air pollution as much as possible, but also to prevent damage to the engine, exhaust gas analyzers working on the above principle are less satisfactory.

In the second system, too, an element is heated by a constant energy supply, but here so much energy is supplied that the member is brought up to a high temperature sufficient to effect a catalytic burning under the supply of air, of the still available combustible components of the exhaust gases. When the composition of the mixture is such that the combustion in the engine takes place below the stoichiometrical point, the mixture being too rich, the temperature of the element, which in this case also is detected by a suitable sensor, will increase with decreasing values of the air to fuel ratio, because there will be an increasing concentration of combustible components Patented July 4, 1972 in the mixture of the exhaust gas and air. Above the stoichiometrical point, there is a plentiful supply of air in the mixture fed to the engine, and consequently the exhaust gas is devoid of any combustible components. The measuring signal supplied by the sensor therefore will vary, starting from a too rich mixture, until the stoichiometrical point is reached but then it will remain constant. This means, however, that it is impossible with the use of such a prior exhaust gas analyzer to adjust the composition of the fuel mixture fed to an internal combustion motor such that it is slightly lean, as, for instance, is desired for Diesel engines. This fact is a disadvantage and is a limitation upon the possibilities of the use of prior exhaust gas analyzers working on the above system.

SUMMARY OF THE INVENTION The present invention is based on the concept that a suitable combination of the two previously described systems makes it possible to eliminate the disadvantages of each of the two systems and to preserve the advantages of both of them. The novel exhaust gas analyzer according to the present invention comprises for that purpose a first element to be heated by a certain energy supply, the end temperature of which is detected by a first sensor, and thus the obtained first measuring signal, being a measure of the thermal conductivity of the exhaust gases flowing along this first element and a second element, likewise heated by a certain supply of energy, such that in operation this second element has a sufliciently high temperature to burn catalytically the combustible components of the exhaust gas/air mixture flowing along the same. As a result, the temperature detected by a second sensor, and thus the second measuring signal, is a measure of the caloric value of the gases to be analyzed while this second measuring signal starting from the stoichiometrical point at decreasing values of the air to fuel ratio increases faster than the first measuring signal under these circumstances. These two obtained measuring signals then are fed to a circuit which applies an output signal that is a measure of the difference of the two measuring signals.

As hereinafter will be shown with reference to the description and drawings, the advantage of the novel system forming the present invention is that in case of a variation of the air to fuel ratio of the mixture as supplied to the engine, from below the stoichiometrical point to above the stoichiometrical point, a continuous unequivocal indication on the measuring instrument is obtained.

A preferred embodiment of the invention comprises a first wire element which is connected in an electric measuring circuit in such a way that the resistance of the element during operation, and thus the (first) output signal of the measuring circuit, is a measure of the thermal conductivity of the exhaust gases to be analyzed and which flow along that element, and further comprises a second wire element connected in a second electric measuring circuit, and which in operation has a temperature such that the combustible components of the exhaust gas/ air mixture to be analyzed and which flow along this second element, are catalytically burned so that the resistance of the second element, and thus the (second) output signals are supplied to a subtracting circuit with of the caloric value of the gases to be analyzed, while the second output signal at decreasing values, starting from the stoichiometrical point, of the air to fuel ratio increases relatively faster than the output signal of the first measuring circuit under these circumstances, and these two output signals are supplied to a subtracting circuit with supplies at its output a measuring signal that is a measure of the difference of the two output signals.

The current flowing through the wire element heats the element so that the end temperature of this measuring wire will have a certain value. The measurement on the basis of the thermal conductivity gives an end temperature of the wire which is influenced by the thermal conductivity of gases surrounding the measuring wire. In the case when the wire element serves to determine the caloric value of the exhaust gases, this temperature is determined by the combustible components in the exhaust gas. The changes in the resistance of the wire elements represent the changes of the temperature thereof.

The two measuring elements advantageously are connected into opposite branches of a Wheatstone bridge circuit, the two other branches of the bridge each containing a reference element in such a way that the same current flows through each measuring element with its associated reference element while the output voltage of the bridge circuit is supplied to an indicator.

'It is, however, also possible that each of the measuring elements with its associated member is connected to a voltage dividing circuit which is connected to a voltage source, the respective taps of the voltage dividing circuits each being connected, via an adjustable resistance, with the input of an operational amplifier acting as an adding amplifier, to the input of which a compensation voltage is supplied through an adjustable resistor.

The complete device may comprise a supply tube for the exhaust gases which are to be analyzed, which tube, through a first calibrated opening, communicates with a first measuring chamber containing the measuring element serving to measure the thermal conductivity while this first measuring chamber also is connected with a suction tube and which supply tube communicates through a second calibrated opening with a second measuring chamber containing the measuring element for the determination of the caloric value of the exhaust gases, the second measuring chamber being likewise connected with the suction tube and communicating, through a third caligrated opening with the outer atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is clarified with reference to the accompanying drawings. In the drawings:

FIGS. la to 10 are graphical representations in which the principle of the invention is clarified;

-FIG. 2a is a circuit diagram of a first illustrative embodiment of the measuring circuit;

FIG. 2b is a graphical representation explaining the effect of the system according to FIG. 2;

FIG. 3a is a circuit diagram of a second illustrative embodiment of measuring circuit;

FIG. 3b is a graphical representation clarifying the eifect of this second embodiment;

FIG. 4 shows schematically one structural realization of the exhaust gas analyzer according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1a shows how in an exhaust gas analyzer, operating on the thermal conductivity principle, the thermal conductivity Wg of the exhaust gas of an internal combustion engine varies as a function of the air to fuel ratio A/F of the mixture fed to the engine. This thermal conductivity Wg, plotted on the Y-axis, is minimal on the stoichiometrical point S, the situation in which the air to fuel ratio has such a value that the combustion in the internal combustion engine proceeds stoichiometrically. Since the resistance of the measuring wire element, and therewith, the output voltage of the measuring bridge, varies in a corresponding way as the thermal conductivity Wg, the indication of the indicating instrument around the stoichiometrical point is not unequivocal; a value of the output voltage Vu denoted by the letter corresponds with two values of the air to fuel ratio A1, A2, respectively, which are below and above the stoichiometrical point respectively.

FIG. lb relates to the system of exhaust gas analysis according to which the combustible components of a mixture of the exhaust gases to be analyzed are catalytically burned under the addition of air. Plotted along the Y-axis is the rise of temperature of the measuring wire element as a function of the air to fuel ratio of the fuel mixture supplied to the engine; a high temperature of the measuring wire element corresponds with a high value of the resistance of the measuring wire element. Above the stoichiometrical point, where a lean mixture (excess of air) is supplied to the engine, the exhause gas does not contain any combustible components and as a consequence no variation of the resistance of the measuring wire and thereby of the output voltage V11 of the relative measuring circuit takes place. It therefore is impossible to adjust, by means of such an analyzer, the fuel system of an engine such that the mixture is a little on the lean side since above the stoichiometrical point the indicator no longer responds to variations of the composition of the mixture.

The invention has the advantage of combining the desirable features of the two known systems. When the measuring circuit using catalytic burning for analysis is realized in such a Way that the output voltage thereof in case of decreasing air to fuel ratios under the stoichiometrical point increases relatively faster than that of the measuring circuit on the basis of the thermal conductivity methodwhich in itself is quite easy-then, when the output voltages of the two measuring circuits are subtracted from each other, the resulting voltage vu -Vu will vary as a function of the air to fuel ratio, in the way as indicated in FIG. 1c. As is shown, there is a continuous variation of the measuring voltage and therefore of the indication of the measuring instrument when the air to fuel ratio A/F is changed, with an unequivocal indication around the stoichiometrical point and a continuous indication above the stoichiometrical point.

FIG. 2 shows the circuit diagram of the electrical circuit of an illustrative exhaust gas analyzer according to the invention, connected as a Wheatstone bridge. The figure shows a direct current source E and the four re sistors R to R which are, in fact:

R is the measuring wire element acting for the measurement based on thermal conductivity;

R is the reference wire element for this measurement;

-R is the reference wire element for the measurement based on catalytical burning;

R is the measuring wire element for this measurement.

FIG. 2b shows the variation of the resistance of the measuring wire element R as a function of the air to fuel ratio A/F. This figure shows also the variation of the voltage V; across the reference resistor R caused by the variation of R; as a function of the air to fuel ratio A/F.

The figure also shows the variation of the measuring resistor R, as a function of the air to fuel ratio and the variation of the voltage V across the resistor R which is caused by the variations of R It appears that the voltage V V measured with the measuring instrument M and shown in the graph of FIG. 2b, varies in the desired way.

FIG. 3a shows a circuit in which an operational amplifier is used. In this circuit:

R is the measuring wire element for the measurement based on thermal conductivity;

R is the reference wire element for this measurement;

R is the measuring wire element for the measurement based on the principle of catalytical burning;

R is the reference wire element.

According to the operational theory of operational amplifiers, when it is assumed that:

the output voltage of the operational amplifier A is equal to:

FIG. 3b shows the variation of the value of R; as a function of the air to fuel ratio and the variation of the voltage V across resistor R and the variation of the value of the resistor R and the voltage V across the resistor R, as a function of the air to fuel ratio A/F. It appears that the output voltage of the operational amplifier 0A, indicated by the measuring instrument M, has the desired variation. By means of the resistor R R and R it is possible to adjust and to calibrate the measuring instrument.

FIG. 4 shows schematically a possible embodiment. The exhaust gases are sucked away through the tube 1 and discharged by a suction pump connected to the tube 2. The tube 1 communicates through the tube 3 with the chamber 4 which on its turn, through the tube 5, is connected with the suction tube 2; furthermore the tube 1 communicates through the tube 6 with the chamber 7 which again through the tube 8 is connected with the suction tube 2 and through the tube 9 communicates with the atmosphere.

The chamber 7 contains the measuring element 10 for the analysis on the principle of catalytic burning of the exhaust gases; the associated reference element 11 lies in the free air. The chamber 4 contains the measuring element 12 for the measurement on the thermal conductivity principle; the associated reference element 13 is likewise in the free air. The tube 6 contains, between the connection of the tube 1 and the chamber 7, a calibrated opening 14; the tube 9 is nearly closed by the calibrated opening 15, while the tube 3 contains the calibrated opening 16.

Through the calibrated opening 14 the exhaust gases flow to the measuring chamber 7 for the catalytic burning, air flowing in through the calibrated opening 15. These openings are such that there is a sufiiciently large concentration of air to make possible the catalytic burning of the combustible components in the exhaust gases. The passage 16 is dimensioned in such a way that the exhaust gas arrives simultaneously in the measuring chambers 4 and 7.

The measuring and reference elements may be connected into any of the circuits as described hereinbefore.

What is claimed as the invention is:

1. An exhaust gas analyzer for internal combustion engines employing mixtures of air and fuel in both stoichiometrical and nonstoichiometn'cal ratios comprising:

(a) a first measuring element positioned to be heated by a certain energy supply and having sensing means for detecting the temperature reached by said first measuring element upon the flow of exhaust gases therealong,

said temperature reached being defined as the end temperature of said first measuring element,

(b) a first reference element positioned remote from said first measuring element so as to be free of the heating effect of said energy supply and said exhaust gases,

(c) said first measuring element and said first reference element being connected in a circuit to provide a first signal which is a measure of the thermal conductivity of said exhaust gases,

(d) a second measuring element positioned to be heated by a certain energy supply such that in operation said second measuring element has a sufficiently high temperature to burn catalytically the combustible components of the air and exhaust gas mixture flowing along the same, said second measuring element having sensing means for detecting the temperature reached by said second measuring element upon the flow of exhaust gases therealong,

said temperature reached being defined as the end temperature of said second measuring element,

(e) a second reference element positioned remote from said second measuring element so as to be free of the heating effect of said energy supply and of said exhaust gases, said second measuring element and said second reference element being connected in a circuit to provide a second signal which is a measure of the caloric value of the gases being analyzed,

said second signal increasing faster than said first signal at decreasing values of said air to fuel ratio starting from said stoichiometric ratio point, and

(f) an output circuit to which is fed said first and second signals, which circuit supplies an output signal that is a measure of the difference between said first and second signals.

2. An exhaust gas analyzer according to claim 1 where- (a) said first measuring and reference elements each are resistance wires connected in a first electric measuring circuit for providing said first signal,

(b) said second measuring and reference elements each are resistance wires connected in a second measuring circuit for providing said second signal, and

(c) said output circuit to which said first and said measuring signals are supplied is a subtracting circuit.

3. An exhaust gas analyzer according to claim 2' wherein said first and said second measuring elements are connected into two opposite branches of a Wheatstone bridge circuit, the two other branches of the bridge respectively comprising said first and said second reference elements, connected in such a way that the same current flows through each measuring element and its associated reference element and further connected to enable the output voltage of the Wheatstone bridge circuit to be supplied to an indicator instrument.

4. An analyzer according to claim 2 wherein each of said measuring elements, with its associated reference element, is connected to form a voltage dividing circuit which is connected to a voltage source, the respective taps of the voltage dividing circuits each being connected, by means of an adjustable resistance, with the input of an operational amplifier acting as an adding amplifier, to the input of which a compensation voltage is supplied through an adjustable resistance.

5. An exhaust gas analyzer as in claim 1 which comprises:

(a) supply tube means having a plurality of conduits for said exhaust gases,

(b) a first and a second calibrated opening in said supply tube means, said first and second calibrated openings being located in separate conduits of said supply tube means,

(c) a first measuring chamber communicating with said supply tube means through said first calibrated open- 8,

(d) said first measuring element being positioned in said first measuring chamber,

(e) a second measuring chamber communicating with said supply tube means through said second calibrated opening,

(f) said second measuring element being positioned in said second measuring chamber,

(g) a suction tube in communication with both said first and said second chambers, and

(h) a third calibrated opening in said suction tube communicating with the atmosphere.

6. A method of analyzing exhaust gases which comprises the steps of:

(a) causing a first portion of exhaust gases to flow into contact with a first measuring element initially heated by a heat source at a temperature below the oxidation temperature of said gases,

(b) measuring the end temperature of said first measuring element in comparison with a reference element free of the exhaust gas flow and producing a first signal corresponding to the thermal conductivity of said gases at said heat source,

(0) causing a second portion of exhaust gases to flow into contact with a second measuring elemient initially heated by a heat source to a temperaturesufliciently high to burn said gases,

((1) measuring the end temperature of said second measuring element in comparison with a reference element free of the exhaust gas flow and producing a second signal corresponding to the caloric value of said burning gases, and

(e) combining said first and second signals to produce an output signal corresponding to the diiference between said first and second signals, and which is indicative of the gas composition being analyzed.

References Cited UNITED STATES PATENTS 10 JOSEPH SCOVRONEK, Primary Examiner R. E. SERWIN, Assistant Examiner US. Cl. X.R.

15 23,254 E, 255 E; 7327 R 

